Superconductor

A magnet levitating above a high-temperature superconductor, cooled with liquid nitrogen. Persistent electric current flows on the surface of the superconductor. This excludes the magnetic field of the magnet (Faraday's law of induction). In effect, the current forms an electromagnet that repels the magnet

A superconductor is a substance that conducts electricity without resistance when it becomes colder than a "critical temperature." At this temperature, electrons can move freely through the material. Superconductors are different from ordinary conductors, even very good ones. Ordinary conductors lose their resistance slowly as they get colder. In contrast, superconductors lose their resistance all at once. This is an example of a phase transition. High magnetic fields destroy superconductivity and restore the normal conducting state.

Normally, a magnet moving by a conductor produces currents in the conductor by electromagnetic induction. But a superconductor actually pushes out magnetic fields entirely by inducing surface currents. Instead of letting the magnetic field pass through, the superconductor acts like a magnet pointing the opposite way, which repels the real magnet. This is called the Meissner effect, and it can be demonstrated by levitating a superconductor over magnets or vice versa.

the Meissner effect discovered by Walter Meissner and Robert Ochsenfeld

1957

theoretical explanation for superconductivity put forward by John Bardeen, Leon Cooper, and John Schrieffer (BCS theory)

1962

the tunneling of superconducting Cooper pairs through insulating barrier predicted

1986

A ceramic superconductor was discovered by Alex Müller and Georg Bednorz. Ceramics are normally insulators. A lanthanum, barium, copper and oxygen compound with a critical temperature of 30K. Opened up the possibilities for new superconductors.